Forming dienes from cyclic ethers and diols, including tetrahydrofuran and 2-methyl-1,4-butanediol

ABSTRACT

Forming a diene includes contacting a reactant including at least one of a cyclic ether and a diol with a heterogeneous acid catalyst to yield a reaction mixture including a diene. The heterogeneous acid catalyst includes at least one of a Lewis acid catalyst, a supported Lewis-acid catalyst, a Brnsted acid catalyst, a solid acid catalyst, a supported phosphoric acid catalyst, and a sulfonated catalyst. The dehydration of cyclic ethers and diols with high selectivity to yield dienes completes pathways for the production of dienes, such as isoprene and butadiene, from biomass in high yields, thereby promoting economical production of dienes from renewable resources.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Application Ser. No. 62/402,238 entitled “USE OF PHOSPHORUS-CONTAINING SOLID CATALYSTS” filed Sep. 30, 2016; U.S. Application Ser. No. 62/410,919 entitled “METHODS OF FORMING ISOPRENE FROM 2-METHYL-1,4-BUTANEDIOL, DERIVATIVES OR COMBINATIONS THEREOF” filed Oct. 21, 2016; U.S. Application Ser. No. 62/410,922 entitled “METHODS OF FORMING DIENES FROM TETRAHYDROFURAN, DERIVATIVES OR COMBINATIONS THEREOF” filed Oct. 21, 2016; U.S. Application Ser. No. 62/414,302 entitled “PHOSPHORUS-CONTAINING SOLID CATALYSTS AND METHODS OF USE THEREOF” filed Oct. 28, 2016; U.S. Application Ser. No. 62/419,202 entitled “METHODS OF FORMING ISOPRENE FROM 2-METHYL-1,4-BUTANEDIOL, DERIVATIVES OR COMBINATIONS THEREOF” filed Nov. 8, 2016; and U.S. Application Ser. No. 62/419,247 entitled “METHODS OF FORMING DIENES FROM TETRAHYDROFURAN, DERIVATIVES, OR COMBINATIONS THEREOF” filed Nov. 8, 2016, all of which are incorporated by reference herein in their entirety.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under DE-SC000104 awarded by the Department of Energy and CHE-1413862 awarded by the National Science Foundation. The government has certain rights in the invention.

BACKGROUND

Some dienes, such as butadiene and isoprene, are used to produce rubbery polymers (e.g., for use in car tires). These dienes are commonly produced from the cracking of naptha and other petroleum-derived precursors. Although these dienes can be produced from renewable resources such as bio-based feedstocks, yields are generally low, making these processes cost prohibitive. In one example, depicted in Scheme 1, furfural can be formed from xylose, a readily available biomass-derived platform molecule. Furfural can then be converted to furan through decarbonylation, followed by hydrogenation to yield tetrahydrofuran. Dehydra-decyclization of tetrahydrofuran yields butadiene, completing the pathway for the production of butadiene from biomass.

Synthetic methods with improved diene yields would make production of dienes from renewable resources economically feasible.

SUMMARY

In a general aspect, forming a diene includes contacting a reactant with a heterogeneous acid catalyst to yield a reaction mixture including a diene, where the reactant includes a cyclic ether or a diol. The heterogeneous acid catalyst includes at least one of a Lewis acid catalyst, a solid Lewis-acid catalyst, a Brønsted acid catalyst, a solid acid catalyst, a supported phosphoric acid catalyst, and a sulfonated catalyst.

Implementations of the general aspect may include one or more of the following features.

The reactant may be derived from biomass. When the reactant is a cyclic ether, the cyclic ether may have tetrahydrofuran skeleton. In some embodiments, the cyclic either is tetrahydrofuran, and the diene is butadiene. A selectivity of the butadiene is typically at least 95%. In some embodiments, the cyclic ether is 2-methyltetrahydrofuran, and the diene is pentadiene. A selectivity of the pentadiene is typically at least 95%. In some embodiments, the cyclic ether is 2,5-dimethyl tetrahydrofuran and the diene is hexadiene. A selectivity of the hexadiene is typically at least 90%. In some embodiments, the cyclic ether is 3-methyltetrahydrofuran, and the diene is isoprene. A selectivity of the isoprene is typically at least 65%. When the cyclic ether is 3-methyltetrahydrofuran, forming the diene may include processing biomass to yield citric acid, itaconic acid, or mesaconic acid, and processing the citric acid, itaconic acid, or mesaconic acid to yield the 3-methyltetrahydrofuran. That is, the 3-methyltetrahydrofuran may be formed from at least one of citric acid, itaconic acid, and mesaconic acid derived from biomass. When the reactant is a diol, the diol may be 2-methyl-1,4-butanediol, and the diene is isoprene. A selectivity of the isoprene is typically at least 70%.

In some implementations, the contacting occurs at a temperature from 100° C. to 600° C., 150° C. to 400° C., 100° C. to 500° C., or 200° C. to 300° C. The contacting may occur at a pressure from 0 psia to 500 psia (34 atm), at a pressure up to 147 psia (10 atm), at pressure from 1 atm to 10 atm, or from 1 atm to 2 atm. In some cases, the contacting occurs in the presence of an inert gas, such as He, Ar, and N₂. In certain cases, the contacting occurs in the vapor phase. Some implementations may include separating the diene from the reaction mixture. The reaction mixture may include unreacted cyclic ether, unreacted diol, or both, and the unreacted cyclic ether, unreacted diol, or both may be contacted with the heterogeneous acid catalyst to yield the diene.

In some implementations, the heterogeneous acid catalyst includes a Lewis acid catalyst, and the Lewis acid catalyst includes at least one of AlCl₃, TiCl₄, FeCl₃, BF₃, SnCl₄, ZnCl₂, ZnBr₂, AMBERLYST-70, SiO₂, Nb₂O₅, MgO, TiO₂, SiO₂—Al₂O₃, CeO₂, and Cr₂O₃.

In some implementations, the heterogeneous acid catalyst includes a Brønsted acid catalyst, and the Brønsted acid catalyst includes at least one of HCl, HBr, HI, HClO₄, HClO₃, HNO₃, H₂SO₄, CH₃COOH, CF₃COOH, and H₃PO₄.

In some implementations, the heterogeneous acid catalyst includes a solid acid catalyst, and the solid acid catalyst includes at least one of a zeolite type catalyst, a substituted-zeolite type catalyst, a heteropolyacid type catalyst, a phosphate type catalyst, a zirconia type catalyst, a celite type catalyst, a metal organic framework type catalyst, a carbon type catalyst, and a sulfonic acid catalyst. Suitable zeolite type catalysts include H-ZSM-5, H-BEA, H-Y, mordenite, ferrierite, chabazite, and self-pillared pentasil. The self-pillared pentasil typically has an average pore size of at least 5 A. Suitable phosphorus-containing zeolite type catalysts include phosphorus-containing MFI, phosphorus-containing MEL, phosphorus-containing BEA, phosphorus-containing FAU, phosphorus-containing MOR, phosphorus-containing FER, phosphorus-containing CHA, and phosphorus-containing self-pillared pentasil. The phosphorus-containing self-pillared pentasil has a ratio of silicon atoms to phosphorus atoms in a range of 1:1 to 1000:1 or 3:1 to 150:1. The phosphorus-containing self-pillared pentasil has rotational intergrowths of single-unit-cell lamellae that lead to repetitive branching nanosheets. The nanosheets have a thickness of about 2 nm and define a network of micropores having a diameter of about 0.5 nm. The phosphorus-containing self-pillared pentasil has a house of cards arrangement defining a network of mesopores having a dimension in a range of 2 nm to 7 nm. The phosphorus-containing zeolite type catalyst is substantially free of aluminum. That is, the phosphorus-containing zeolite type catalyst may include less than 5 wt %, less than 4 wt %, less than 3 wt %, less than 2 wt %, or less than 1 wt % of aluminum. Suitable substituted zeolite type catalyst include Sn, Ge, Ga, B, Ti, Fe, and Zr.

Suitable heteropolyacid type catalysts include H₃PW₁₂O₄₀, H₃SiW₁₂O₄₀, H₅AlW₁₂O₄₀, H₆CoW₁₂O₄₀, H₃PMo₁₂O₄₀, H₃SiMo₁₂O₄₀, and Cs⁺ substituted heteropolyacid type catalyst.

Suitable phosphate type catalysts include niobium phosphate (NbOPO₄), zirconium phosphate (ZrO₂—PO₄), siliconiobium phosphate (Nb—P—Si—O), tricalcium phosphate (Ca₃(PO₄)₂), lithium phosphate (Li₃PO₄), and lithium sodium phosphate (Li₃NaP₂O₇).

Suitable zirconia type catalysts include SO₃—ZrO₂, SiO₂—ZrO₂, Zeolites-ZrO₂, Al₂O₃—ZrO₂, ZrO₂, and WO_(x)—ZrO₂.

Suitable celite type catalysts include P-CELITE.

Suitable metal organic framework catalysts include MTh 101.

Suitable carbon type catalysts include activated carbon, sulfated carbon, and SO₃H-functionalized carbon.

Suitable sulfonated catalysts include NAFION and AMBERLYST.

Advantages of processes described herein include dehydration of cyclic ethers and diols with high selectivity to yield dienes, there by completing pathways for the production of dienes, such as isoprene and butadiene, from biomass in high yields. These synthetic methods facilitate economical production of dienes from renewable resources.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts a reaction diagram for the dehydration of 2-methyl-1,4-butanediol (MBDO) to isoprene at 200° C. and 1 atm. Intermediates in the dehydration reaction include 3-methyltetrahydrofuran (3-MTHF), 2-methyl-3-butene-1-ol, and 3-methyl-3-butene-1-ol (MBEO).

FIG. 2 shows a process scheme for the production of isoprene from MBDO with continuous recycling of 3-MTHF and MBDO.

FIG. 3 depicts of high throughput pulsed flow reactor (HTPFR).

FIG. 4 shows butadiene yield and selectivity formed by the dehydration of tetrahydrofuran (THF) for various catalysts.

FIG. 5 shows THF conversion and butadiene selectivity for various catalysts.

FIG. 6 shows isoprene yield and isoprene selectivity formed by the dehydration of MBDO various catalysts.

DETAILED DESCRIPTION

Disclosed herein are processes and methods for the dehydration of cyclic ethers and diols over heterogeneous acid catalysts to produce dienes. Examples of suitable cyclic ethers include furans (compounds with a tetrahydrofuran skeleton), including tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MTHF), 3-methyltetrahydrofuran (3-MTHF), and 2,5 dimethyltetrahydrofuran (2,5-DMTHF). Examples of suitable diols include butanediols, such as 2-methyl-1,4-butanediol (MBDO). The particular diene produced depends on the reactant (e.g., the cyclic ether or diol) and the reaction conditions (e.g., temperature or pressure). In some embodiments, dienes produced include 1,3-butadiene (“butadiene”), 2-methyl-1,3-butadiene (isoprene), pentadiene, and hexadiene.

One embodiment, depicted in Scheme 2, includes a process for the dehydra-decyclization of tetrahydrofuran (THF) over a heterogeneous acid catalyst to yield 1,3-butadiene and water. The THF may be obtained from any biomass-derived source, such as biomass-derived furan. This process, which completes the pathway for the production of butadiene from biomass depicted in Scheme 1, results in a high yield of 1,3-butadiene, with a selectivity of at least 95%.

Another embodiment, depicted in Scheme 3, includes a process for the dehydration of 2-methyltetrahydrofuran (2-MTHF), which can be readily derived from biomass, over a heterogeneous acid catalyst to yield pentadiene with a selectivity of at least 95%.

Yet another embodiment, depicted in Scheme 4, includes a process for the dehydration of 2,5 dimethyltetrahydrofuran (2,5-DMTHF), which can be readily derived from biomass, over a heterogeneous acid catalyst to yield hexadiene with a selectivity of at least 90%.

Another embodiment, depicted in Scheme 5, includes a process for the dehydration of 3-methyltetrahydrofuran (3-MTHF), which can be readily derived from biomass, over a heterogeneous acid catalyst to yield isoprene. The selectivity to isoprene of this reaction is at least 65%.

Still another embodiment, depicted in Scheme 6, includes a process for dehydration of 2-methyl-1,4-butanediol (MBDO), which can be readily derived from biomass, over a heterogeneous acid catalyst to yield isoprene with a selectivity of at least 70%. In some examples, MBDO is produced from biomass-derived citric acid, itaconic acid, or mesaconic acid. Thus, disclosed methods complete the pathway for the production of isoprene in high yields from biomass.

FIG. 1 shows a calculated reaction energy diagram for MBDO dehydration. Isoprene is the thermodynamically preferred product at 200° C. and 1 atm pressure. At temperatures lower than 150° C., 3-MTHF becomes the preferred product. As such, greater selectivity for isoprene can be achieved at temperatures of at least about 150° C. The upper temperature limit (e.g., 400° C. or 600° C.) for producing isoprene is governed by bond-forming reactions of isoprene (e.g., Diels-Alder of two isoprene molecules to form limonene), coking reactions, and bond-breaking reactions, all of which may be favored at higher temperatures.

Dehydration reactions disclosed herein can be carried out with a variety of catalysts at a variety of temperatures, pressures, and space velocities (reactant volumetric flow rate per volume of catalyst). Suitable catalysts include acid catalysts, such as those listed in Table 1. In some embodiments, dehydration reactions described herein take place at an elevated temperature (relative to room temperature) over a heterogeneous acid catalyst. In certain embodiments, dienes can be produced from cyclic ethers or diols without adding water to the reaction, simplifying and reducing the cost of the process.

TABLE 1 Acid catalyst classes, types, and examples suitable for dehydration of cyclic ethers and diols to yield dienes. Class Type Example Lewis Acid (L- L-Acids AlCl₃, TiCl₄, FeCl₃, BF₃, SnCl₄, Acid) Catalysts ZnCl₂, ZnBr₂, Amberlyst-70 Solid L-Acids SiO₂, Al₂O₃, Nb₂O₅, MgO TiO₂, SiO₂—Al₂O₃, CeO₂, Cr₂O₃ BrØnsted Acid B-Acids HCl, HBr, HI, HClO₄, HClO₃, (B-Acid) HNO₃, H₂SO₄, CH₃COOH, Catalysts CF₃COOH, H₃PO₄ Solid Acid Zeolites (Z) H-ZSM-5, H-BEA, H—Y, Catalysts Mordenite, Ferrierite, Chabazite, Self-Pillared Pentasil (SPP), phosphorus-containing zeolites (e.g., P-BEA, P-MFI) Substituted Zeolites (Sub.) Sn, Ge, Ti, Fe, Zr, P Heteropolyacids (HPAs) H₃PW₁₂O₄₀, H₅AlW₁₂O₄₀, H₆CoW₁₂O₄₀, H₃SiW₁₂O₄₀, H₃PMo₁₂O₄₀, H₃SiMo₁₂O₄₀ (Cs⁺ substituted HPAs) Phosphates (PO₄ ³⁻) Niobium phosphate (NbOPO₄), Zirconium phosphate (ZrO₂—PO₄), Siliconiobium phosphate (Nb—P—Si—O) Tricalcium phosphate (Ca₃(PO₄)₂) Lithium phosphate (Li₃PO₄) Lithium sodium phosphate (Li₃NaP₂O₇) Zirconias (ZrO₂) SO₃—ZrO₂, SiO₂—ZrO₂, Zeolites-ZrO₂, Al₂O₃—ZrO₂, ZrO₂, WO_(x)—ZrO2 Celite ® P-Celite ® Metal Organic Framework Metal organic framework (MOF) Carbon (C) Activated carbon, sulfated carbon (SO₃H-functionalized carbon)

Other suitable catalysts include a supported phosphoric acid catalysts.

Other suitable catalysts include self-pillared pentasil (SPP). Phosphorus-containing-self-pillared pentasil (P-SPP) is a silica-based, self-pillared, hierarchical (containing both micropore and mesopore) zeolitic material. In some embodiments, the ratio of silicon to phosphorus in suitable P-SPP catalysts is typically in a range of 1 to 5. In certain embodiments, the ratio of silicon to phosphorus in suitable P-SPP catalysts is 50 or less, 500 or less, or 1000 or less. Examples of ranges for the ratio of silicon to phosphorus in suitable P-SPP catalysts include 1 to 1000, 5 to 500, and 5 to 50 (e.g., 27). P—SPP can be synthesized by a direct hydrothermal method using tetrabutylphosponium hydroxide (TBPOH) as an organic structure directing agent, with TBPOH providing phosphorus for the P-SPP. The rotational intergrowths of single-unit-cell lamellae leads to repetitive branching nanosheets. The nanosheets can be about 2 nm thick and can contain a network of micropores having a diameter of about 0.5 nm. The house-of-cards arrangement of the nanosheets creates a network mesopores having a diameter in a range of about 2 nm to about 7 nm. As used herein, a micropore has a diameter of less than about 2 nm, and a mesopores has a diameter between about 2 nm and about 50 nm.

In some embodiments, other phosphorus-containing zeolites (“P-zeolites”) can be utilized to form dienes from cyclic ethers and diols. Suitable P-zeolites include, for example P-BEA, P-MFI and P-MEL, P-FAU, P-MOR, P-FER, P-CHA, and P-SPP. One example, P-BEA can be prepared by impregnating a zeolite having a BEA framework with phosphoric acid, as described in Fan W. et al. (2016) Renewable p-Xylene from 2,5-Dimethylfuran and Ethylene Using Phosphorus-containing Zeolite Catalysts. ChemCatChem (2016) (DOI: 10.1002/cctc.201601294). In some cases, suitable P-zeolites are free or substantially free of aluminum. A P-zeolite that is “substantially free” of aluminum can include, for example, less than 5 wt %, less than 4 wt %, less than 3 wt %, less than 2 wt %, or less than 1 wt % of aluminum.

In some embodiments, P-Celite® prepared by impregnating Celite® with phosphoric acid can be used to form dienes from cyclic ethers and diols. P-Celite® may be made according to Fan W., et al. (2016) Renewable p-Xylene from 2,5-Dimethylfuran and Ethylene Using Phosphorus-containing Zeolite Catalysts. ChemCatChem (2016) (DOI: 10.1002/cctc.201601294).

In some embodiments sulfonated (e.g., sulfonic acid) catalysts can be utilized. Suitable sulfonic acid catalysts (also referred to as sulfonate catalysts) include NAFION and AMBERLYST.

The dehydration of cyclic ethers and diols to yield dienes can be carried out using any appropriate temperature. In some embodiments, the reaction can be carried out in the vapor phase. In some embodiments, the reaction can be carried out at temperatures of not less than 100° C., not less than 150° C., or not less than 200° C. In some embodiments the reaction can be carried out at a temperature of not greater than 600° C., not greater than 500° C., not greater than 400° C., not greater than 350° C., or not greater than 300° C. In some embodiments, the reaction can be carried out at a temperature from 100° C. to 600° C., from 150° C. to 400° C., from 100° C. to 500° C., or from 200° C. to 300° C. For selective formation of isoprene from 3-MTHF, suitable temperatures are from 150° C. to 350° C.

The dehydration of cyclic ethers or diols can be carried out at any appropriate pressure. In some embodiments the reaction pressure can be not less than vacuum (0 psia) or not less than 1 atm. In some embodiments, the reaction pressure can be 10 atm or less, or 2 atm or less. In some embodiments, the reaction pressure can be from vacuum (0 psia) to 500 psia, or from 1 atm to 2 atm. Low pressures may yield selective formation of certain dienes, such as isoprene, due to favorable thermodynamic conditions. Thus, in some cases, pressures less than 10 atm may be selected.

The dehydration of cyclic ethers or diols can be carried out at any appropriate space velocity. In some embodiments, space velocity can be chosen to obtain single-pass conversion of the cyclic either or diol that is less than 100%. In some embodiments, space velocity can be chosen such that 100% conversion is obtained. In certain embodiments, space velocity can be considerably higher than that necessary to obtain 100% conversion of the cyclic ether or diol.

Dehydration reactions described herein may be carried out with or without an inert carrier gas (e.g., He, Ar, N₂, etc.) added to or contacted with the cyclic ether or diol prior to the reactants entering a catalytic reactor with the catalyst.

Because the selectivity of the dehydration reactions disclosed herein to a particular diene may decrease with increasing conversion of the reactant (i.e., cyclic ether or diol), it may be useful to perform the process using a recycle stream of the reactant. In some embodiments, as depicted in FIG. 2, a reaction intermediate, such as 3-MTHF in the formation of isoprene from MBDO, may be recycled with the cyclic ether or diol.

In some embodiments, zeolites may be useful in the dehydration of cyclic ethers and diols to dienes (e.g., MBDO to isoprene). In particular, zeolites such as microporous aluminosilicates with pore sizes of not less than about 5 A, not less than about 4 A, or not less than about 3 A may promote diene formation over that of other products (e.g., isoprene formation over 3-MTHF) based on the size of the reactant and product molecules.

This disclosure is further illustrated by the following examples. The particular examples, materials, amounts, and procedures are to be interpreted broadly in accordance with the scope and spirit of the disclosure as set forth herein.

Examples Dehydra-Decyclization of Tetrahydrofurans

The following catalysts were tested for THF dehydra-decyclization: CeO₂ (Sigma Aldrich), H-ZSM-5 Zeolite (Zeolyst CBV28014 SiO₂/Al₂O₃ ratio=280), H-Y Zeolite (Zeolyst CBV760, SiO₂/Al₂O₃=60), H-BEA Zeolite (Zeolyst CBV811C-300 SiO₂/Al₂O₃ ratio=360), tin-BEA Zeolite (Sn-BEA) (Chang C-C, Wang Z, Dornath P, Je Cho H, & Fan W (2012) Rapid synthesis of Sn-Beta for the isomerization of cellulosic sugars. RSC Advances 2(28):10475-10477), ZrO₂ (Sigma Aldrich), phosphorous-self-pillared pentasil (P-SPP), phosphorous-celite (P-Celite) (Hong Je Cho, Limin Ren, Vivek Vattipalli, Yu-Hao Yeh, Nicholas Gould, Bingjun Xu, Raymond J. Gorte, Raul Lobo, Paul J. Dauenhauer, Michael Tsapatsis, and Wei Fan, Renewable p-Xylene from 2, 5-Dimethylfuran and Ethylene Using Phosphorus-Containing Zeolite Catalysts ChemCatChem 9 (3), 398-402, 2017) MgO (Alfa Aesar), tricalcium phosphate (TCP, Sigma Aldrich), SiO₂—Al₂O₃(Sigma Aldrich), activated carbon (Sigma Aldrich), metal organic framework catalyst (MOF) (e.g., MIL 101), niobium oxide (Nb₂O₅, Sigma Aldrich), phosphotungstic acid (PTA, H₃PW₁₂O₄₀). All catalysts were pressed, crushed, and sieved to a particle size of 0.5 to 1 mm. With the exception of MOF, all catalysts were pre-treated under flowing He at 400° C. for 1 h. MOF was pre-treated under flowing He at 150° C. for 1 h.

Dehydration reactions were performed in a high throughput pulsed flow reactor (HTPFR), such as HTPFR 100 depicted in FIG. 3. HTPFR 100 includes a glass reactor (i.e., inlet liner) 102 coupled to a gas chromatograph (Aglient 7890A) via conduit 104. Glass reactor 102 is packed with quartz wool 106 (7.5 mg and 15 mg) and catalyst 108 (5-200 mg). Carrier gas is provided to glass reactor 102 via inlet 110, and gas exits via split vent 112. Reactant is provided to HTPFR 100 through injection needle 114.

Experiments were performed at reaction temperatures of 200 to 400° C. The space velocity was controlled by adjusting the carrier gas (He, 99.999%) flow rate to the split vent. Space velocities of 10-89 s⁻¹ were tested with the reactor, where space velocity is defined as follows:

$S = {{{space}\mspace{14mu} {velocity}} = {{\frac{F_{He}}{V_{C}}\lbrack = \rbrack}\min^{- 1}}}$

where F_(He) is the flow rate of He through in sccm min⁻¹ and V_(C) is the volume of the catalyst bed. He pressure was kept constant at 30 psig. Each experiment was performed by injecting 1 uL of reactant (cyclic ether or diol) into the reactor followed by immediate separation and quantification of the products.

Reaction products were quantified by separating on a column (Agilent Plot-Q, 30 m, 0.32 mm ID, 20 μm film thickness; temperature program: 40° C. for 2 min, 10° C./min to 270° C., hold 10 min) and detecting with a Polyarc/FID. The Polyarc (Activated Research Company) allows for calibration-free quantitative analysis of hydrocarbons because the FID signal area is proportional to the moles of each compound. Diene selectivity is defined as the ratio of moles of C from the diene divided by the total moles of C from the diene plus the moles of C from all reaction by-products.

${{Diene}\mspace{14mu} {Selectivity}} = \frac{{Diene}\;\left\lbrack {{mol}\mspace{14mu} C} \right\rbrack}{{{Diene}\mspace{11mu}\left\lbrack {{mol}\mspace{14mu} C} \right\rbrack} + {\sum{By}} - {{products}\mspace{11mu}\left\lbrack {{mol}\mspace{14mu} C} \right\rbrack}}$

Diene yield and reactant conversion are defined as follows:

${{Diene}\mspace{14mu} {Yield}} = \frac{{Diene}\mspace{11mu}\left\lbrack {{mol}\mspace{14mu} C} \right\rbrack}{\sum{{Reactant}\mspace{14mu} {and}\mspace{14mu} {{Products}\mspace{11mu}\left\lbrack {{mol}\mspace{14mu} C} \right\rbrack}}}$ ${{Reactant}\mspace{14mu} {Conversion}} = {1 - \frac{{Reactant}\mspace{11mu}\left\lbrack {{mol}\mspace{14mu} C} \right\rbrack}{\sum{{Reactant}\mspace{14mu} {and}\mspace{14mu} {{Products}\mspace{11mu}\left\lbrack {{mol}\mspace{14mu} C} \right\rbrack}}}}$

where Reactant and Products [mol C] refers to all reaction products including the diene and by-products as well as the reactant.

FIG. 4 shows a summary of the catalytic data obtained for dehydra-decyclization of THF with various catalysts. Detailed catalytic data for the dehydration of THF to butadiene over fifteen different catalysts are presented in Tables 2-16. Data for 2-methyltetrahydrofuran (2-MTHF) and 2,5-dimethyltetrahydrofuran (2,5-DMTHF) to butadiene are presented in Tables 17 and 18.

TABLE 2 Tetrahydrofuran dehydra-decyclization to butadiene using CeO₂ Catalyst CeO₂ Mass (mg) 100 Space Temperature Velocity Butadiene Butadiene THF (° C.) (s⁻¹) Selectivity Yield Conversion 200 89 1.0000 0.0000 0.0000 200 10 0.5355 0.0001 0.0001 300 89 1.0000 0.0000 0.0000 300 10 0.6157 0.0000 0.0000 400 89 0.3613 0.0003 0.0008 400 10 0.4085 0.0004 0.0010

TABLE 3 Tetrahydrofuran dehydra-decyclization to butadiene using H-ZSM-5 zeolite Catalyst H-ZSM-5 Mass (mg) 61.8 Space Temperature Velocity Butadiene Butadiene THF (° C.) (s⁻¹) Selectivity Yield Conversion 200 89 0.8085 0.0209 0.0259 200 10 0.5603 0.0215 0.0383 300 89 0.4598 0.1439 0.3109 300 10 0.1880 0.1203 0.6281 400 89 0.1546 0.1126 0.7204 400 10 0.1281 0.1266 0.9875

TABLE 4 Tetrahydrofuran dehydra-decyclization to butadiene using H—Y zeolite Catalyst H—Y Mass (mg) 45.2 Space Temperature Velocity Butadiene Butadiene THF (° C.) (s⁻¹) Selectivity Yield Conversion 200 89 0.9598 0.1001 0.1043 200 10 0.8807 0.2903 0.3296 300 89 0.6589 0.2760 0.4189 300 10 0.4917 0.4683 0.9524 400 89 0.4063 0.2310 0.5686 400 10 0.3162 0.3012 0.9525

TABLE 5 Tetrahydrofuran dehydra-decyclization to butadiene using P-SPP Catalyst P-SPP Mass (mg) 42.5 Space Temperature Velocity Butadiene Butadiene THF (° C.) (s⁻¹) Selectivity Yield Conversion 200 89 0.4646 0.0004 0.0008 200 10 0.8612 0.0016 0.0019 300 89 0.9379 0.0079 0.0084 300 10 0.9776 0.0250 0.0256 400 89 0.9626 0.0308 0.0320 400 10 0.9731 0.1084 0.1114

TABLE 6 Tetrahydrofuran dehydra-decyclization to butadiene using ZrO₂. Catalyst ZrO₂ Mass (mg) 200 Space Temperature Velocity Butadiene Butadiene THF (° C.) (s⁻¹) Selectivity Yield Conversion 200 89 0.0000 0.0000 0.0000 200 10 1.0000 0.0001 0.0001 300 89 1.0000 0.0056 0.0056 300 10 0.7570 0.0126 0.0166 400 89 0.6231 0.0122 0.0196 400 10 0.6041 0.0288 0.0476

TABLE 7 Tetrahydrofuran dehydra-decyclization to butadiene using tricalcium phosphate Catalyst TCP Mass (mg) 111.7 Space Temperature Velocity Butadiene Butadiene THF (° C.) (s⁻¹) Selectivity Yield Conversion 200 89 0.0167 0.0000 0.0011 200 10 0.0204 0.0000 0.0012 300 89 0.0172 0.0001 0.0040 300 10 0.0491 0.0003 0.0069 400 89 0.1139 0.0002 0.0018 400 10 0.0425 0.0007 0.0105

TABLE 8 Tetrahydrofuran dehydra-decyclization to butadiene using SiO₂—Al₂O₃ Catalyst SiO₂•Al₂O₃ Mass (mg) 200 Space Temperature Velocity Butadiene Butadiene THF (° C.) (s⁻¹) Selectivity Yield Conversion 200 89 0.8631 0.0076 0.0088 200 10 0.8688 0.0202 0.0233 300 89 0.4728 0.1754 0.3710 300 10 0.4640 0.4640 1.0000 400 89 0.4833 0.4833 1.0000 400 10 0.2522 0.2522 1.0000

TABLE 9 Tetrahydrofuran dehydra-decyclization to butadiene using H-BEA zeolite Catalyst H-BEA Mass (mg) 45.1 Space Temperature Velocity Butadiene Butadiene THF (° C.) (s⁻¹) Selectivity Yield Conversion 200 89 0.6813 0.1809 0.2655 200 10 0.5765 0.5017 0.8702 300 89 0.2849 0.2234 0.7840 300 10 0.3982 0.3873 0.9725 400 89 0.2036 0.1803 0.8857 400 10 0.3512 0.3512 1.0000

TABLE 10 Tetrahydrofuran dehydra-decyclization to butadiene using magnesium oxide (MgO) Catalyst MgO Mass (mg) 96 Space Temperature Velocity Butadiene Butadiene THF (° C.) (s⁻¹) Selectivity Yield Conversion 200 89 0.0000 0.0000 0.0219 200 10 0.0000 0.0000 0.0001 300 89 0.5000 0.0000 0.0000 300 10 0.5733 0.0000 0.0000 400 89 0.0232 0.0000 0.0010 400 10 0.1513 0.0002 0.0012

TABLE 11 Tetrahydrofuran dehydra-decyclization to butadiene using phosphotungstic acid (PTA), H₃PW₁₂O₄₀ Catalyst PTA Mass (mg) 136 Space Temperature Velocity Butadiene Butadiene THF (° C.) (s⁻¹) Selectivity Yield Conversion 200 89 0.9042 0.0070 0.0077 200 10 0.8298 0.0156 0.0188 300 89 0.7143 0.0308 0.0431 300 10 0.5704 0.0492 0.0862 400 89 0.6062 0.0735 0.1213 400 10 0.5369 0.1018 0.1896

TABLE 12 Tetrahydrofuran dehydra-decyclization to butadiene using activated carbon (C) Catalyst C Mass (mg) 17.7 Space Temperature Velocity Butadiene Butadiene THF (° C.) (s⁻¹) Selectivity Yield Conversion 200 89 0.0000 0.0000 0.0000 200 10 0.0000 0.0000 0.0000 300 89 0.0000 0.0000 0.0001 300 10 0.1913 0.0000 0.0001 400 89 0.6331 0.0001 0.0002 400 10 0.0773 0.0001 0.0012

TABLE 13 Tetrahydrofuran dehydra-decyclization to butadiene using metal organic framework (MOF) (MIL 101) Catalyst MOF Mass (mg) 25 Space Temperature Velocity Butadiene Butadiene THF (° C.) (s⁻¹) Selectivity Yield Conversion 150 89 0.4447 0.0615 0.1383 150 10 0.5049 0.0952 0.1885 175 89 0.4453 0.0245 0.0549 175 10 0.6482 0.0485 0.0748 200 89 0.4041 0.0036 0.0088 200 10 0.1924 0.0073 0.0379

TABLE 14 Tetrahydrofuran dehydra-decyclization to butadiene using niobia (Nb₂O₅) Catalyst Nb₂O₅ Mass (mg) 132.5 Space Temperature Velocity Butadiene Butadiene THF (° C.) (s⁻¹) Selectivity Yield Conversion 200 89 0.6622 0.0002 0.0003 200 10 0.6217 0.0003 0.0004 300 89 0.7707 0.0209 0.0271 300 10 0.7412 0.0197 0.0266 400 89 0.7636 0.0405 0.0530 400 10 0.6945 0.0370 0.0533

TABLE 15 Tetrahydrofuran dehydra-decyclization to butadiene using Sn-BEA Zeolite Catalyst Sn-BEA Mass (mg) 44.1 Space Temperature Velocity Butadiene Butadiene THF (° C.) (s⁻¹) Selectivity Yield Conversion 200 89 0.8857 0.0001 0.0002 200 10 0.9443 0.0005 0.0005 300 89 0.8455 0.0009 0.0011 300 10 0.6842 0.0046 0.0067 400 89 0.5676 0.0035 0.0061 400 10 0.5215 0.0170 0.0327

TABLE 16 Tetrahydrofuran dehydra-decyclization to butadiene using P-Celite ® Catalyst P-Celite ® Mass (mg) 41.1 Space Temperature Velocity Butadiene Butadiene THF (° C.) (s⁻¹) Selectivity Yield Conversion 200 89 0.5015 0.0002 0.0003 200 10 0.6741 0.0004 0.0006 300 89 0.8442 0.0018 0.0022 300 10 0.9023 0.0044 0.0049 400 89 0.9199 0.0119 0.0130 400 10 0.9149 0.0372 0.0407

TABLE 17 2-methyltetrahydrofuran dehydra-decyclization to pentadiene using P-SPP Catalyst P-SPP Mass (mg) 42 Space Temperature Velocity Pentadiene Pentadiene 2-MTHF (° C.) (s⁻¹) Selectivity Yield Conversion 200 89 0.9788 0.0169 0.0172 200 10 0.9915 0.0538 0.0542 300 89 0.9914 0.0822 0.0829 300 10 0.9726 0.2903 0.2985 400 89 0.9471 0.2104 0.2221 400 10 0.9265 0.7009 0.7565

TABLE 18 2,5-dimethyltetrahydrofuran dehydra- decyclization to hexadiene using P-SPP Catalyst P-SPP Mass (mg) 42 Space Temperature Velocity Hexadiene Hexadiene 2,5 DMTHF (° C.) (s⁻¹) Selectivity Yield Conversion 200 89 0.9902 0.0547 0.0553 200 10 0.9687 0.4061 0.4192 300 89 0.9830 0.2053 0.2088 300 10 0.9096 0.6934 0.7624 400 89 0.9252 0.4075 0.4404 400 10 0.8101 0.8101 1.0000

A variety of solid acid catalysts were tested for the dehydra-decyclization of tetrahydrofuran to butadiene, including: phosphorous acid impregnated self-pillared pentasil (P-SPP), phosphotungstic acid supported on MCM-41 (PWA), tin framework substituted BEA zeolite (Sn-BEA), amorphous silica alumina (SiAl), and Al framework zeolite (ZSM-5). Catalysts were tested in a packed bed flow reactor operated at 250° C. and a weight hourly space velocity of 1 g THF g catalyst⁻¹ hf⁻¹, with a THF partial pressure of 5 torr. FIG. 5 shows selectivity to butadiene (line plot) and THF conversion (bar graph) for these catalysts. While ZSM-5 provides the highest conversion of THF, it resulted in the lowest selectivity to butadiene. Conversely, P-SPP affords the highest selectivity to butadiene but with the lowest conversion level of THF. PWA provides an optimal intermediate with regards to butadiene yield; the conversion to THF was comparable to ZSM-5 with a relatively high selectivity to butadiene of about 90%.

Preparation and Dehydration of 2-Methyl-1,4-Butanediol

Liquid-phase hydrogenation reactions were performed in 100 mL high pressure reactors (model 4598HPHT, Parr Instrument Co.) equipped with Hastelloy C-276 internals, a magnetic stirrer with gas-entrainment propeller, liquid sampling port, and electronic pressure gauge. 2-methyl-1,4-butanediol was prepared by a previously established method (Spanjers, C. S.; Schneiderman, D. K.; Wang, J. Z.; Wang, J.; Hillmyer, M. A.; Zhang, K.; Dauenhauer, P. J. ChemCatChem 2016, DOI: 10.10). Briefly, 40 g of itaconic acid (Sigma Aldrich) was added to a 100 mL Parr reactor with 20 mL deionized water. To the mixture, 2 g 10 wt. % Pd/C (Sigma Aldrich) was added. The mixture was heated to 220° C. under 140 bar H₂ for 3 days. The reactor was subsequently cooled, de-pressurized, and 2.5 g 5 wt. % Ru/C (Sigma Aldrich) was added. The mixture was re-heated to 120° C. under 140 bar H₂ for 3 days. The reaction product was filtered and purified in a rotary evaporator to yield 95% pure MBDO.

Dehydration reactions were performed in a high throughput pulsed flow reactor (HTPFR), such as that described with respect to FIG. 3. Experiments were performed at reaction temperatures of 175 to 400° C. The space velocity was controlled by adjusting the carrier gas (He, 99.999%) flow rate to the split vent. Space velocities of 9-400 s′ were tested with the reactor, where space velocity is defined previously herein. He pressure was kept constant at 30 psig.

Each experiment was performed by injecting 1 μL of pure MBDO into the reactor followed by immediate separation and quantification of the products. Reaction products were quantified by separating on a column (Agilent Plot-Q, 30 m, 0.32 mm ID, 20 μm film thickness; temperature program: 40° C. for 2 min, 10° C./min to 270° C., hold 10 min) and detecting with a Polyarc/FID. The Polyarc (Activated Research Company) allows for calibration-free quantitative analysis of hydrocarbons because the FID signal area is proportional to the moles of each compound.

Isoprene selectivity is defined previously herein as diene selectivity. The reaction by-products exclude 3-MTHF, because this compound is an intermediate in the production of isoprene and can be recycled. 3-MTHF yield is defined analogously to diene yield.

The following catalysts were tested for 2-methyl-1,4-butanediol dehydration to isoprene: CeO₂ (Sigma Aldrich), H-ZSM-5 Zeolite (Zeolyst CBV28014 SiO₂/Al₂O₃ ratio=280), H-Y Zeolite (Zeolyst CBV760, SiO₂/Al₂O₃=60), H-BEA Zeolite (Zeolyst CBV811C-300 SiO₂/Al₂O₃ ratio=360), ZrO₂ (Sigma Aldrich), phosphorous-self-pillared pentasil (P-SPP), MgO (Alfa Aesar), tricalcium phosphate (TCP, Sigma Aldrich), SiO₂.Al₂O₃(Sigma Aldrich), activated carbon (Sigma Aldrich), metal organic framework catalyst (MOF), niobium oxide (Nb₂O₅, Sigma Aldrich), phosphotungstic acid (H₃PW₁₂O₄₀, Sigma Aldrich), Sn-BEA Zeolite (Chang C-C, Wang Z, Dornath P, Je Cho H, & Fan W (2012) Rapid synthesis of Sn-Beta for the isomerization of cellulosic sugars. RSC Advances 2(28):10475-10477) and P-Celite (Hong Je Cho, Limin Ren, Vivek Vattipalli, Yu-Hao Yeh, Nicholas Gould, Bingjun Xu, Raymond J. Gorte, Raul Lobo, Paul J. Dauenhauer, Michael Tsapatsis, and Wei Fan, Renewable p-Xylene from 2, 5-Dimethylfuran and Ethylene Using Phosphorus-Containing Zeolite Catalysts ChemCatChem 9 (3), 398-402, 2017)). All catalysts were pressed, crushed, and sieved to a particle size of 0.5 to 1 mm. With the exception of MOF MTh 101, all catalysts were pre-treated under flowing He at 400° C. for about 1 hr. MOF MIL 101 was pre-treated under flowing He at 150° C. for 1 hr.

FIG. 6 shows a summary of the catalytic data obtained for the illustrative catalysts that were tested. Detailed catalytic data for the dehydration of MBDO to isoprene over fifteen different catalysts are presented in Tables 19-33. Data for the conversion of 3-MTHF to isoprene over P-SPP is presented in Table 34.

TABLE 19 2-methyl-1,4-butanediol dehydration to isoprene using CeO₂ Catalyst CeO₂ Mass (mg) 100 Temper- Space ature Velocity Isoprene Isoprene 3-MTHF MBDO (° C.) (s⁻¹) Selectivity Yield Yield Conversion 200 201 0.0000 0.0000 0.0013 0.0050 200 101 0.0000 0.0000 0.0010 0.0069 200 52 0.0000 0.0000 0.0009 0.0055 200 22 0.0000 0.0000 0.0007 0.0055 250 201 0.0396 0.0003 0.0113 0.0184 250 101 0.0353 0.0003 0.0125 0.0209 250 52 0.0385 0.0003 0.0120 0.0195 250 22 0.0199 0.0001 0.0090 0.0159 300 201 0.3480 0.0086 0.0182 0.0428 300 101 0.4020 0.0132 0.0237 0.0565 300 52 0.4155 0.0136 0.0231 0.0558 300 22 0.3179 0.0105 0.0241 0.0572 350 201 0.4524 0.0241 0.0257 0.0789 350 101 0.4890 0.0286 0.0382 0.0967 350 52 0.4410 0.0255 0.0403 0.0982 350 22 0.4129 0.0259 0.0464 0.1092 400 201 0.4582 0.0453 0.0916 0.1906 400 101 0.3730 0.0393 0.1279 0.2333 400 52 0.3376 0.0338 0.1303 0.2305 400 22 0.4210 0.0308 0.1320 0.2052

TABLE 20 2-methyl-1,4-butanediol dehydration to isoprene using H-ZSM-5 zeolite Catalyst H-ZSM-5 Zeolite, SiO₂/Al₂O₃ = 280 Mass (mg) 75 Space 3- Temperature Velocity Isoprene Isoprene MTHF MBDO (° C.) (s⁻¹) Selectivity Yield Yield Conversion 200 80 0.4731 0.0407 0.9140 1.0000 200 9 0.3416 0.0606 0.8225 1.0000 250 80 0.1937 0.1096 0.4341 1.0000 250 9 0.0581 0.0581 0.0000 1.0000 300 80 0.0974 0.0811 0.1671 1.0000 300 9 0.0466 0.0466 0.0000 1.0000 350 80 0.0617 0.0576 0.0656 1.0000 350 9 0.0489 0.0489 0.0000 1.0000

TABLE 21 2-methyl-1,4-butanediol dehydration to isoprene using H—Y zeolite Catalyst H—Y Zeolite, SiO₂/Al₂O₃ = 60 Mass (mg) 65 Space 3- Temperature Velocity Isoprene Isoprene MTHF MBDO (° C.) (s⁻¹) Selectivity Yield Yield Conversion 200 80 0.0941 0.0300 0.6811 1.0000 200 9 0.0319 0.0232 0.2715 1.0000 250 80 0.0702 0.0223 0.6825 1.0000 250 9 0.0384 0.0256 0.3338 1.0000 300 80 0.0735 0.0273 0.6293 1.0000 300 9 0.0173 0.0162 0.0591 1.0000 350 80 0.0404 0.0253 0.3739 1.0000 350 9 0.0234 0.0234 0.0000 1.0000

TABLE 22 2-methyl-1,4-butanediol dehydration to isoprene using P-SPP Catalyst P-SPP Mass (mg) 7 Space 3- Temperature Velocity Isoprene Isoprene MTHF MBDO (° C.) (s⁻¹) Selectivity Yield Yield Conversion 200 202 0.6516 0.0094 0.3768 0.3913 250 202 0.7369 0.0321 0.4079 0.4515 300 202 0.7061 0.0539 0.4821 0.5585 350 202 0.6798 0.0670 0.5871 0.6857 400 202 0.6361 0.0804 0.6264 0.7528 400 103 0.6398 0.1037 0.7374 0.8995 400 44 0.6560 0.1208 0.8023 0.9865

TABLE 23 2-methyl-1,4-butanediol dehydration to isoprene using ZrO₂ Catalyst ZrO₂ Mass (mg) 200 Space 3- Temperature Velocity Isoprene Isoprene MTHF MBDO (° C.) (s⁻¹) Selectivity Yield Yield Conversion 200 89 0.0692 0.0004 0.0683 0.0735 200 10 0.0659 0.0004 0.0808 0.0867 250 89 0.1704 0.0052 0.1782 0.2086 250 10 0.1802 0.0053 0.2501 0.2795 300 89 0.2828 0.0356 0.4191 0.5451 300 10 0.2482 0.0261 0.6057 0.7109 350 89 0.3310 0.0701 0.5992 0.8110 350 10 0.3186 0.0587 0.8157 1.0000 400 89 0.3015 0.0816 0.7116 0.9824 400 10 0.2831 0.0605 0.7864 1.0000

TABLE 24 2-methyl-1,4-butanediol dehydration to isoprene using tricalcium phosphate Catalyst Tricalcium Phosphate Mass (mg) 100 Space 3- Temperature Velocity Isoprene Isoprene MTHF MBDO (° C.) (s⁻¹) Selectivity Yield Yield Conversion 200 201 0.0000 0.0000 0.0007 0.0023 200 101 0.0000 0.0000 0.0004 0.0020 200 52 0.0000 0.0000 0.0005 0.0020 200 22 0.0000 0.0000 0.0007 0.0024 250 201 0.0000 0.0000 0.0004 0.0019 250 101 0.0000 0.0000 0.0006 0.0021 250 52 0.0000 0.0000 0.0008 0.0026 250 22 0.0000 0.0000 0.0011 0.0030 300 201 0.0000 0.0000 0.0006 0.0017 300 101 0.0000 0.0000 0.0008 0.0024 300 52 0.0000 0.0000 0.0011 0.0030 300 22 0.0000 0.0000 0.0016 0.0035 350 201 0.0000 0.0000 0.0016 0.0032 350 101 0.0000 0.0000 0.0025 0.0047 350 52 0.0000 0.0000 0.0026 0.0046 350 22 0.0053 0.0000 0.0033 0.0061 400 201 0.0056 0.0000 0.0113 0.0167 400 101 0.0079 0.0000 0.0159 0.0200 400 52 0.0042 0.0000 0.0203 0.0289 400 22 0.0076 0.0000 0.0261 0.0312

TABLE 25 2-methyl-1,4-butanediol dehydration to isoprene using SiO₂•Al₂O₃ Catalyst SiO₂•Al₂O₃ Mass (mg) 50 Space 3- Temperature Velocity Isoprene Isoprene MTHF MBDO (° C.) (s⁻¹) Selectivity Yield Yield Conversion 200 160 0.4698 0.0103 0.9781 1.0000 225 160 0.5347 0.0202 0.9621 1.0000 250 160 0.5159 0.0387 0.9251 1.0000 275 160 0.5687 0.0549 0.9035 1.0000 300 160 0.4933 0.0586 0.8812 1.0000 325 160 0.4127 0.0695 0.8316 1.0000 350 160 0.3554 0.0776 0.7818 1.0000 375 160 0.2941 0.0905 0.6923 1.0000 400 160 0.2685 0.1098 0.5910 1.0000 300 17 0.3413 0.1985 0.4185 1.0000 300 41 0.3621 0.1366 0.6227 1.0000 300 81 0.3660 0.0885 0.7582 1.0000 300 120 0.3641 0.0544 0.8505 1.0000

TABLE 26 2-methyl-1,4-butanediol dehydration to isoprene using H-BEA zeolite Catalyst H-BEA Zeolite SiO₂/Al₂O₃ = 360 Mass (mg) 8 Space 3- Temperature Velocity Isoprene Isoprene MTHF MBDO (° C.) (s⁻¹) Selectivity Yield Yield Conversion 200 400 0.4058 0.0072 0.2481 0.2658 200 44 0.4886 0.0117 0.4937 0.5177 250 400 0.3963 0.0097 0.1745 0.1990 300 400 0.3543 0.0149 0.2376 0.2797 300 44 0.3166 0.0279 0.5778 0.6659 350 400 0.2968 0.0254 0.3832 0.4686 400 400 0.2530 0.0449 0.6334 0.8110 400 44 0.2194 0.0700 0.6571 0.9761

TABLE 27 2-methyl-1,4-butanediol dehydration to isoprene using magnesium oxide (MgO) Catalyst MgO Mass (mg) 100 Space 3- Temperature Velocity Isoprene Isoprene MTHF MBDO (° C.) (s⁻¹) Selectivity Yield Yield Conversion 200 89 0.0000 0.0000 0.0048 0.0048 200 10 0.0000 0.0000 0.0006 0.0015 250 89 0.0000 0.0000 0.0032 0.0032 250 10 0.0000 0.0000 0.0035 0.0053 300 89 0.0000 0.0000 0.0025 0.0049 300 10 0.0000 0.0000 0.0052 0.0082 350 89 0.0000 0.0000 0.0049 0.0106 350 10 0.0174 0.0002 0.0154 0.0282

TABLE 28 2-methyl-1,4-butanediol dehydration to isoprene using phosphotungstic acid (PTA), H₃PW₁₂O₄₀ Catalyst PTA Mass (mg) 100 Space 3- Temperature Velocity Isoprene Isoprene MTHF MBDO (° C.) (s⁻¹) Selectivity Yield Yield Conversion 200 89 0.1075 0.0037 0.9658 1.0000 200 10 0.2715 0.0155 0.9429 1.0000 250 89 0.1064 0.0050 0.9534 1.0000 250 10 0.1404 0.0109 0.9226 1.0000 300 89 0.2344 0.0150 0.9360 1.0000 300 10 0.1662 0.0152 0.8867 1.0000 350 89 0.3365 0.0406 0.8793 1.0000 350 10 0.1305 0.0222 0.8296 1.0000 400 89 0.3492 0.0682 0.8048 1.0000 400 10 0.1009 0.0239 0.7632 1.0000

TABLE 29 2-methyl-1,4-butanediol dehydration to isoprene using activated carbon (C) Catalyst Activated carbon Mass (mg) 20 Space 3- Temperature Velocity Isoprene Isoprene MTHF MBDO (° C.) (s⁻¹) Selectivity Yield Yield Conversion 200 89 0.0516 0.0005 0.0423 0.0529 200 10 0.0412 0.0004 0.0151 0.0241 250 89 0.0486 0.0003 0.0093 0.0156 250 10 0.0260 0.0002 0.0354 0.0428 300 89 0.0103 0.0001 0.0167 0.0226 300 10 0.0195 0.0002 0.0663 0.0743 350 89 0.0232 0.0002 0.0187 0.0252 350 10 0.0086 0.0001 0.0827 0.0989

TABLE 30 2-methyl-1,4-butanediol dehydration to isoprene using metal organic framework (MIL 101) Catalyst MOF Mass (mg) 20 Space 3- Temperature Velocity Isoprene Isoprene MTHF MBDO (° C.) (s⁻¹) Selectivity Yield Yield Conversion 200 89 0.0059 0.0003 0.4034 0.4494 200 10 0.0028 0.0002 0.9163 0.9925 250 89 0.0245 0.0014 0.8226 0.8788 250 10 0.0300 0.0009 0.9679 0.9994 300 89 0.1059 0.0087 0.7696 0.8515 300 10 0.1037 0.0077 0.9185 0.9960 175 89 0.0000 0.0000 0.1142 0.1449 175 10 0.0000 0.0000 0.1445 0.1720 150 89 0.0000 0.0000 0.0356 0.0514 150 10 0.0000 0.0000 0.0366 0.0492

TABLE 31 2-methyl-1,4-butanediol dehydration to isoprene using niobia (Nb₂O₅) Catalyst Nb₂O₅ Mass (mg) 130 Space 3- Temperature Velocity Isoprene Isoprene MTHF MBDO (° C.) (s⁻¹) Selectivity Yield Yield Conversion 200 89 0.0212 0.0002 0.1299 0.1394 200 10 0.0484 0.0002 0.1501 0.1543 250 89 0.3807 0.0064 0.2807 0.2974 250 10 0.4121 0.0082 0.3757 0.3956 300 89 0.4226 0.0315 0.4830 0.5575 300 10 0.4145 0.0342 0.7116 0.7940 350 89 0.3467 0.0313 0.7597 0.8500 350 10 0.3136 0.0221 0.9295 1.0000 400 89 0.0317 0.0048 0.8492 1.0000 400 10 0.0333 0.0066 0.8031 1.0000

TABLE 32 2-methyl-1,4-butanediol dehydration to isoprene using P-Celite ® Catalyst P-Celite ® Mass (mg) 35 Space 3- Temperature Velocity Isoprene Isoprene MTHF MBDO (° C.) (s⁻¹) Selectivity Yield Yield Conversion 200 160 0.4818 0.0081 0.3077 0.3246 200 17 0.3921 0.0065 0.2948 0.3114 250 160 0.4674 0.0152 0.2661 0.2987 250 17 0.4710 0.0158 0.6028 0.6362 300 160 0.3700 0.0165 0.4023 0.4468 300 17 0.4576 0.0253 0.8165 0.8717 350 160 0.3690 0.0294 0.4336 0.5133 350 17 0.5091 0.0502 0.8578 0.9565 400 160 0.4510 0.0588 0.4713 0.6016 400 17 0.5542 0.1032 0.8116 0.9978

TABLE 33 2-methyl-1,4-butanediol dehydration to isoprene using Sn-BEA zeolite Catalyst Sn-BEA Mass (mg) 47.1 Space 3- Temperature Velocity Isoprene Isoprene MTHF MBDO (° C.) (s⁻¹) Selectivity Yield Yield Conversion 200 89 0.3243 0.0038 0.9884 1.0000 200 10 0.3322 0.0035 0.9896 1.0000 250 89 0.2994 0.0087 0.9710 1.0000 250 10 0.3730 0.0081 0.9784 1.0000 300 89 0.2613 0.0162 0.9381 1.0000 300 10 0.3063 0.0137 0.9552 1.0000 350 89 0.2820 0.0230 0.9183 1.0000 350 10 0.2924 0.0210 0.9283 1.0000 400 89 0.3071 0.0315 0.8975 1.0000 400 10 0.2800 0.0359 0.8717 1.0000

TABLE 34 3-Methyltetrahydrofuran dehydration to isoprene using P-SPP Catalyst P-SPP Mass (mg) 45 Temperature Space Velocity S Y 3-MTHF (° C.) (s⁻¹) (Isoprenes) (Isoprenes) Conversion 200 89 0.0796 0.0010 0.0130 200 10 0.5493 0.0057 0.0103 250 89 0.2029 0.0044 0.0218 250 10 0.6294 0.0199 0.0317 300 89 0.3207 0.0073 0.0229 300 10 0.7703 0.0493 0.0640 350 89 0.4036 0.0114 0.0284 350 10 0.7919 0.1032 0.1303 400 89 0.5338 0.0211 0.0395 400 10 0.8230 0.1668 0.2027

Thus, embodiments of methods of forming dienes from cyclic ethers and diols are disclosed. The implementations described above and other implementations are within the scope of the following claims. One skilled in the art will appreciate that the present disclosure can be practiced with embodiments other than those disclosed. The disclosed embodiments are presented for purposes of illustration and not limitation. 

1. A method of forming a diene, the method comprising: contacting a reactant comprising at least one of a cyclic ether and a diol with a heterogeneous acid catalyst to yield a reaction mixture comprising a diene, wherein the heterogeneous acid catalyst comprises at least one of a Lewis acid catalyst, a solid Lewis-acid catalyst, a Brønsted acid catalyst, a solid acid catalyst, a supported phosphoric acid catalyst, and a sulfonated catalyst.
 2. The method of claim 1, wherein the reactant is derived from biomass.
 3. The method of claim 1, wherein the reactant comprises a cyclic ether.
 4. The method of claim 3, wherein the cyclic ether has a tetrahydrofuran skeleton.
 5. The method of claim 4, wherein the cyclic either comprises tetrahydrofuran and the diene comprises butadiene.
 6. The method of claim 5, wherein a selectivity of the butadiene is at least 95%.
 7. The method of claim 3, wherein the cyclic ether comprises 2-methyltetrahydrofuran and the diene comprises pentadiene.
 8. The method of claim 7, wherein a selectivity of the pentadiene is at least 95%.
 9. The method of claim 3, wherein the cyclic ether comprises 2,5-dimethyl tetrahydrofuran and the diene comprises hexadiene.
 10. The method of claim 9, wherein a selectivity of the hexadiene is at least 90%.
 11. The method of claim 3, the cyclic ether comprises 3-methyltetrahydrofuran, and the diene comprises isoprene.
 12. The method of claim 11, wherein a selectivity of the isoprene is at least 65%.
 13. The method of claim 11, further comprising processing biomass to yield an acid comprising least one of citric acid, itaconic acid, and mesaconic acid, and processing the acid to yield the 3-methyltetrahydrofuran.
 14. The method of claim 11, wherein the 3-methyltetrahydrofuran is derived from at least one of citric acid, itaconic acid, and mesaconic acid.
 15. The method of claim 1, wherein the reactant comprises a diol.
 16. The method of claim 15, wherein the diol comprises 2-methyl-1,4-butanediol, and the diene comprises isoprene.
 17. The method of claim 16, wherein a selectivity of the isoprene is at least 70%.
 18. The method of claim 1, wherein the contacting occurs at a temperature from 100° C. to 600° C., 150° C. to 400° C., 100° C. to 500° C., or 200° C. to 300° C.
 19. The method of claim 1, wherein the contacting occurs at a pressure from 0 psia to 500 psia (34 atm), at a pressure up to 147 psia (10 atm), at pressure from 1 atm to 10 atm, or from 1 atm to 2 atm.
 20. The method of claim 1, wherein the contacting occurs in the presence of an inert gas.
 21. The method of claim 20, wherein the inert gas comprises at least one of He, Ar, and N₂.
 22. The method of claim 1, wherein the contacting occurs in the vapor phase.
 23. The method of claim 1, further comprising separating the diene from the reaction mixture.
 24. The method of claim 1, wherein the reaction mixture comprises unreacted cyclic ether, unreacted diol, or both, and further comprising contacting the unreacted cyclic ether, unreacted diol, or both with the heterogeneous acid catalyst to yield the diene.
 25. The method of claim 1, wherein the heterogeneous acid catalyst comprises a Lewis acid catalyst, and the Lewis acid catalyst comprises at least one of AlCl₃, TiCl₄, FeCl₃, BF₃, SnCl₄, ZnCl₂, ZnBr₂, AMBERLYST-70, SiO₂, Nb₂O₅, MgO, TiO₂, SiO₂—Al₂O₃, CeO₂, and Cr₂O₃.
 26. The method of claim 1, wherein the heterogeneous acid catalyst comprises a Brønsted acid catalyst, and the Brønsted acid catalyst comprises at least one of HCl, HBr, HI, HClO₄, HClO₃, HNO₃, H₂SO₄, CH₃COOH, CF₃COOH, and H₃PO₄.
 27. The method of claim 1, wherein the heterogeneous acid catalyst comprises a solid acid catalyst, and the solid acid catalyst comprises at least one of a zeolite type catalyst, a substituted-zeolite type catalyst, a heteropolyacid type catalyst, a phosphate type catalyst, a zirconia type catalyst, a celite type catalyst, a metal organic framework type catalyst, a carbon type catalyst, and a sulfonic acid catalyst.
 28. The method of claim 27, wherein the solid acid catalyst comprises a zeolite type catalyst.
 29. The method of claim 28, wherein the zeolite type catalyst comprises at least one of H-ZSM-5, H-BEA, H-Y, mordenite, ferrierite, chabazite, and self-pillared pentasil.
 30. The method of claim 29, wherein the zeolite type catalyst comprises self-pillared pentasil.
 31. The method of claim 28, wherein the zeolite type catalyst has an average pore size of at least 5 A.
 32. The method of claim 28, wherein the zeolite type catalyst comprises a phosphorus-containing zeolite type catalyst.
 33. The method of claim 32, wherein the phosphorus-containing zeolite type catalyst comprises at least one of phosphorus-containing MFI, phosphorus-containing MEL, phosphorus-containing BEA, phosphorus-containing FAU, phosphorus-containing MOR, phosphorus-containing FER, phosphorus-containing CHA, and phosphorus-containing self-pillared pentasil.
 34. The method of claim 33, wherein the phosphorus-containing zeolite type catalyst comprises phosphorus-containing self-pillared pentasil, and the phosphorus-containing self-pillared pentasil has a ratio of silicon atoms to phosphorus atoms in a range of 1:1 to 1000:1 or 3:1 to 150:1.
 35. The method of claim 33, wherein the phosphorus-containing zeolite type catalyst comprises phosphorus-containing self-pillared pentasil, and the phosphorus-containing self-pillared pentasil has rotational intergrowths of single-unit-cell lamellae that lead to repetitive branching nanosheets.
 36. The method of claim 35, wherein the nanosheets have a thickness of about 2 nm and define a network of micropores having a diameter of about 0.5 nm.
 37. The method of claim 33, wherein the phosphorus-containing self-pillared pentasil has a house of cards arrangement defining a network of mesopores having a dimension in a range of 2 nm to 7 nm.
 38. The method of claim 32, wherein the phosphorus-containing zeolite type catalyst is substantially free of aluminum.
 39. The method of claim 38, wherein the phosphorus-containing zeolite type catalyst comprises less than 5 wt %, less than 4 wt %, less than 3 wt %, less than 2 wt %, or less than 1 wt % of aluminum.
 40. The method of claim 27, wherein the zeolite type catalyst comprises a substituted-zeolite type catalyst.
 41. The method of claim 40, wherein the substituted zeolite type catalyst comprises at least one of Sn, Ge, Ga, B, Ti, Fe, and Zr.
 42. The method of claim 27, wherein the heterogeneous acid catalyst comprises a heteropolyacid type catalyst, and the heteropolyacid type catalyst comprises at least one of H₃PW₁₂O₄₀, H₃SiW₁₂O₄₀, H₅AlW₁₂O₄₀, H₆CoW₁₂O₄₀, H₃PMo₁₂O₄₀, H₃SiMo₁₂O₄₀, and Cs⁺ substituted heteropolyacid type catalyst.
 43. The method of claim 27, wherein the heterogeneous acid catalyst comprises a phosphate type catalyst, and the phosphate type catalyst comprises at least one of niobium phosphate (NbOPO₄), zirconium phosphate (ZrO₂—PO₄), siliconiobium phosphate (Nb—P—Si—O), tricalcium phosphate (Ca₃(PO₄)₂), lithium phosphate (Li₃PO₄), and lithium sodium phosphate (Li₃NaP₂O₇).
 44. The method of claim 27, wherein the heterogeneous acid catalyst comprises a zirconia type catalyst, and the zirconia type catalyst comprises at least one of SO₃—ZrO₂, SiO₂—ZrO₂, Zeolites-ZrO₂, Al₂O₃—ZrO₂, ZrO₂, and WO_(x)—ZrO₂.
 45. The method of claim 27, wherein the heterogeneous acid catalyst comprises a celite type catalyst, and the celite type catalyst comprises P-CELITE.
 46. The method of claim 27, wherein the heterogeneous acid catalyst comprises a metal organic framework catalyst.
 47. The method of claim 46, wherein the metal organic framework catalyst comprises MIL
 101. 48. The method of claim 27, wherein the heterogeneous acid catalyst comprises a carbon type catalyst, and the carbon type catalyst comprises at least one of activated carbon, sulfated carbon, and SO₃H-functionalized carbon.
 49. The method of claim 1, wherein the heterogeneous acid catalyst comprises a sulfonated catalyst, and the sulfonated catalyst comprises at least one of NAFION and AMBERLYST. 